This invention relates to a velocity control apparatus for an elevator in which an electric motor is controlled by the use of an A.C. power source of variable voltage and variable frequency.
A three-phase induction motor is structurally stout, and has another advantage of easy maintenance. An apparatus in which the three-phase induction motor is energized with an A.C. power source of variable voltage and variable frequency, whereby a velocity control substantially equal to that of a D.C. motor is effected over a wide range, is disclosed in, e.g., the official gazette of Japanese Laid-open Patent Application No. 56-132275.
In this regard, the three-phase induction motor can be expressed by an equivalent circuit shown in FIG. 1. Referring to the figure, numeral 1 generally designates the three-phase induction motor, numerals 11 and 12 terminals which are connected to a power source (not shown), and numeral 13 a primary winding which consists of a reactance component of value x.sub.1 and a resistance component of value r.sub.1. Numeral 14 designates a secondary winding, which consists of a reactance component of value x.sub.2 and a resistance component of value r.sub.2 /s which is inversely proportional to a slip s. Shown at numeral 15 is an exciting circuit one end of which is connected between the primary winding 13 and the secondary winding 14.
Now, letting v.sub.1 denote a primary voltage applied across the terminals 11 and 12, w.sub.O a primary frequency across them, i.sub.1 a primary current flowing through the primary winding 13, i.sub.g an exciting current flowing through the exciting circuit 15, i.sub.2 a secondary current flowing through the secondary winding 14, E.sub.2 a secondary induced voltage, s the slip, P.sub.O output power, and T a torque, the following equations of relations hold: EQU i.sub.2 =s E.sub.2 /r.sub.2 ( 1) EQU P.sub.O =i.sub.2.sup.2 (1-s)r.sub.2 /s=E.sub.2 (1-s)i.sub.2( 2) EQU K=E.sub.2 /W.sub.O ( 3) EQU w=w.sub.O (1-s) (4) EQU P.sub.O =K w i.sub.2 ( 5) EQU T=P.sub.O /w=K i.sub.2 ( 6)
It is accordingly understood that, assuming K to be constant, the torque T changes in proportion to the secondary current i.sub.2.
On the other hand, the three-phase A.C. power source of variable voltage and variable frequency is usually controlled so that the ratio between the voltage and the frequency may become constant. EQU That is, V.sub.1 /w.sub.O =constant (7)
FIGS. 2 and 3 show a prior-art control apparatus which employs a variable-voltage variable-frequency power source. Referring to the figures, numeral 21 designates a three-phase induction motor which raises and lowers a cage 22. Numeral 23 indicates load detection means for detecting the load of the three-phase induction motor 21, and specifically used here is a tachometer generator which senses the rotational frequency of the induction motor and generates a velocity signal V.sub.T. Numeral 24 indicates a velocity command unit which generates a velocity command signal V.sub.P, numeral 25 a comparator which compares the velocity command signal V.sub.P and the velocity signal V.sub.T so as to provide the difference signal V.sub.S between them, numeral 26 an adder which adds the difference signal V.sub.S and the velocuty signal V.sub.T, numeral 27 a function generator which generates a frequency command signal F corresponding to the added result of the adder and also generates a voltage command signal V so as to have the relation of a straight line (a) shown in FIG. 3 as a function of the frequency command signal F, numeral 28 a reference sinusoidal wave generator which issues a command on the basis of the frequency command signal F and the velocity command signal V so that a three-phase alternating current of sinusoidal wave may be provided, and numeral 29 an inverter which supplies a three-phase alternating current of variable voltage and variable frequency on the basis of the command of the reference sinusoidal wave generator 28.
In the control apparatus of the above arrangement, when the velocity command signal V.sub.P is generated by the velocity command unit 24, the function generator 27 is fed with the signal through the comparator 25 as well as the adder 26, to deliver the frequency command signal F and the voltage command signal V. These signals change the primary voltage V.sub.1 and primary frequency w.sub.O of the three-phase induction motor 21 which are the output voltage and frequency of the inverter 29, respectively, as indicated by the straight line (a) in FIG. 3. That is, the primary voltage V.sub.1 is set at a value V.sub.O when the primary frequency w.sub.O is zero, whereupon it is rectilinearly increased with the increase in the primary frequency w.sub.O. The three-phase induction motor 21 increases or decreases its rotational frequency in accordance with the primary frequency w.sub.O.
When the three-phase induction motor 21 is subjected to a heavy load, the primary current i.sub.1 increases. As a result, a voltage drop across the primary winding 13 increases to lower the secondary induced voltage E.sub.2. The relation between the secondary induced voltage E.sub.2 and the primary frequency w.sub.O on this occasion becomes as indicated by a straight line (b) in FIG. 3, the gradient of which is smaller than that of the straight line (a). On the other hand, in case of a light load, the primary current i.sub.1 has a small value, so that the voltage drop across the primary winding 13 is small, and the secondary induced voltage E.sub.2 becomes a value close to the primary voltage V.sub.1. The relation between the secondary induced voltage E.sub.2 and the primary frequency w.sub.O on this occasion becomes as indicated by a straight line (c) in FIG. 3, the gradient of which is somewhat smaller than that of the straight line (a).
Thus, in the case of the heavy load, the decrease of the constant K is great in view of Equation (4). Consequently, in view of Equation (6), the secondary current i.sub.2 becomes a large value because a component for compensating the decrease of the constant K flows in addition to a magnitude required for generating the torque T corresponding to the heavy load. The increase of the secondary current i.sub.2 results in increase in the output current of the inverter. Since the inverter 29 is usually constructed of semiconductor elements such as transistors or thyristors, the increase of the current has led to the drawback that the capacities of the semiconductor elements are increased to render the inverter expensive.